1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device having high brightness.
2. Discussion of Related Art
A great need has arisen recently for thin film transistor liquid crystal display devices (TFT LCD) having wide-angle images, for use in such applications as portable televisions, notebook computers, etc. However, the TFT LCD's in use today have a problem in that their contrast ratio depends on the viewing-angle. In order to overcome this problem, a number of approaches have been proposed, including, for example, a twisted nematic LCD having a mounted optical compensator, and a multi-domain LCD, among others. However, it is not expected that the contrast ratio and color shifting dependence on the viewing angle will be adequately solved by such LCDs.
Another proposal discloses an in-plane switching liquid crystal display device. It is suggested that such a device will have a wider viewing angle, as disclosed, for example, in the JAPAN DISPLAY 92 P457, Japanese Patent Unexamined Publication No. 7-36058, Japanese Patent Unexamined Publication No. 7-225338, and ASIA DISPLAY 95 P707.
FIGS. 1 and 2A-2B show a conventional in-plane switching mode liquid crystal display device. As shown in these figures, the alignment directions of the substrates 10, 11 are approximately perpendicular to the gate bus line 1 formed on the first substrate 10 to align the liquid crystal molecules of the liquid crystal layer 19. A first polarizer 25 having a polarization axis parallel to the gate bus line 1 is attached to the substrate 10, and a second polarizer 26 having polarization axis direction parallel to the alignment direction of the alignment layers 21a, 21b is attached to the second substrate 11. Data (or pixel) electrodes 5 and common electrodes 6 are formed in the direction perpendicular to the gate bus lines 1 and the data bus lines 2. The common bus line 7 is parallel to the gate bus line 1. A thin film transistor, having a gate electrode 4 connected to the gate bus line 1 and whose source/drain electrodes 3 are connected to the data bus line 2 and the pixel electrode 5, is formed at the cross region of the gate bus line 1 and the data bus line 2. The liquid crystal display device also includes a shielding layer 8 such as black matrix on the second substrate 11, a color filter layer 17, and passivation layers 16a, 16b.
When no voltage is applied, the liquid crystal molecules are obliquely aligned in the direction of the data and common electrodes 5, 6 along the alignment direction. When voltage is applied, the liquid crystal molecules are rotated and aligned parallel to the gate bus line 1 because of a horizontal electric field parallel to the gate bus line 1. The rotation of the liquid crystal molecules in turn controls the transmittance through the liquid crystal layer 19.
In this in-plane switching mode liquid crystal display device, since the liquid crystal molecules are switching parallel to the surface of the substrate 10, 11, the viewing-angle dependence problem is solved and contrast ratio is improved.
If the liquid crystal is a negative type (N-type) liquid crystal, with dielectric anisotropy .DELTA..epsilon.&lt;0, the liquid crystal molecules 20 in the liquid crystal layer 19 are aligned perpendicular to the electrodes 5, 6 over the entire liquid crystal layer 19, when no voltage is applied, as shown in FIG. 2A. When voltage is applied, the liquid crystal molecules are rotated and aligned perpendicular to the direction of the field applied between the electrodes 5, 6 because of .DELTA..epsilon.&lt;0. Since the first and second alignment layers 21a, 21b are made of organic materials, such as, for example, indium tin oxide (ITO), having anchoring energy higher than the turning effect of the liquid crystal molecules due to the horizontal electric field, the only rotated liquid crystal molecules are those in the vicinity of the middle plane of the liquid crystal layer 19 and not those in the vicinity of the first and second substrates 10, 11. As a result, the liquid crystal molecules 29 are aligned as shown in FIG. 2B.
FIGS. 3A-3B show an in-plane switching mode liquid crystal display device having a positive type (P-type) liquid crystal and a birefringent anisotropy .DELTA..epsilon.&lt;0. When no voltage is applied, as shown in FIG. 3A, the liquid crystal molecules 20 are aligned approximately parallel to the data and common electrodes 5, 6. When voltage is applied, as shown in FIG. 3B, the liquid crystal molecules are rotated and aligned parallel to the direction of the electric field. At this time, however, only liquid crystal molecules 20 in the vicinity of the middle of the liquid crystal layer 19 are rotated, excluding those molecules near the first and second substrates 10, 11, due to the presence of an organic material-coated layer 21a, 21b.
In the conventional in-plane switching liquid crystal display device described above, the liquid crystal molecules are rotated only in the vicinity of the middle plane of the liquid crystal layer 19, so that light transmittance is controlled using the birefringence of the transmitting material. Thus, insufficient light passes through the liquid crystal layer causing image quality deterioration.